Encapsulation is a process in which tiny particles or droplets are surrounded by a coating to impart many useful properties to small capsules. In a relatively simplistic form, a microcapsule is a small sphere with a uniform wall around it. The material inside the microcapsule is referred to as the core, internal phase, or fill, whereas the wall is sometimes called a shell, coating, or membrane. Most microcapsules have diameters between a few microns to a few millimeters. The core may be a crystal, a jagged adsorbent particle, an emulsion, a suspension of solids, or a suspension of smaller microcapsules. The microcapsule even may have multiple walls. The reasons for microencapsulation are countless. In some cases, the core must be isolated from its surroundings, as in isolating vitamins from the deteriorating effects of oxygen, retarding evaporation of a volatile core, improving the handling properties of a sticky material, or isolating a reactive core from chemical attack. In other cases, the objective is not to isolate the core completely but to control the rate at which it leaves the microcapsule, as in the controlled release of drugs or pesticides. The problem may be as simple as masking the taste or odor of the core, or as complex as increasing the selectivity of an adsorption or extraction process.
The technique of microencapsulation depends on the physical and chemical properties of the material to be encapsulated: complex coacervation, centrifugal extrusion, vibrational nozzle, spray-drying, interfacial polycondensation, interfacial cross-linking, in-situ polymerization, etc.
The efforts towards thermal energy storage (TES) and sustainable energy technologies have been intensified over the past decades. Phase change materials (PCMs) can be directly used or consumed as a component of the composite-like energy storage materials (ESM) in TES applications like solar energy utilization, energy conserving in buildings, thermal insulation, thermal adaptable textile materials, etc. because they allow large amounts of heat to be stored during their melting and to be released during their solidifying process. A phase-change material is a substance with a high heat of fusion which, melting and solidifying at a certain temperature, is capable of storing and releasing large amounts of energy. Heat is absorbed or released when the material changes from solid to liquid and vice versa; thus, PCMs are classified as latent heat storage (LHS) units. Micron size dispersion of PCMs in the form of microparticles, microcapsules or PCM impregnated into open-cell foam-like supporting material make them more usable than the traditional block PCM because of: (i) protecting the PCM against the influences of the outside environment, (ii) increasing the heat-transfer area, (iii) permitting the core PSM to withstand changes in volume, as the phase change occurs and allowing small and portable TES system.
Many types of PCMs such as salt hydrates, paraffins, and fatty acids have been investigated. Among the studied PCMs, paraffins (alkanes) have been often used as a latent heat energy storage material (LHESM) with the advantages of high enthalpy of phase change, small segregation of components, small changes in structure and volume during repeated phase transitions (less than 10 v/v %), negligible super- and sub-cooling, low vapor pressure, self-nucleating behavior, very few safety constraints, high chemical stability, insolubility in water, biocompatibility, good recyclability and low cost. They have attractive thermal properties for different applications such as thermal adaptable fibers, thermal insulation building materials, heat exchangers in air conditioning and water heating systems.
The supporting material in the composite-like ESM is often made of high-density polyethylene or polypropylene, polyacrylamide, ceramics, silica powder or wood fiberboards, poly(methyl methacrylate), poly(ethylene oxide), poly(ethylene terephthalate), silk fibroin-chitosan.
Since PCMs transform between solid to liquid and vice versa in thermal cycling, encapsulation naturally become the obvious storage choice. Other than the form stability and leakage resistance microencapsulated bulk PCMs promise additional advantages: (i) no need for additional storage container, thus reducing the cost of TES systems; (ii) minimizing the thermal resistance caused by PCM storage container; (iii) easily fabricated in desired shapes and dimensions; (iv) possibility to cut the ECM into arbitrary shapes without leakage. Most of the above ESM composites are prepared by immersing of the micronized PCM into supporting material by its direct incorporation at the mixing stage of material production.
Additional wide application field of PCMs is the fluid piping heat-transfer systems. In such conventional systems, thermal energy is transferred by the sensible heat of a single-phase working fluid, being proportional to the source/sink temperature difference. Because the systems are often operated with small temperature differences, the single-phase fluid must be pumped at a high-volume flow rate. As a result, the system consumes a large amount of pumping power. The use of PCM particles suspended in a single-phase working fluid (making so called slurry) provides additional thermal capacity from the latent heat associated with the solid/liquid phase change. This enhancement is due to a combination of four factors: (i) the often higher thermal conductivity of the added particles, (ii) the increased microconvection due to the particles, (iii) a higher effective specific heat during the phase change process and, (iv) the greater temperature difference that is maintained as the phase change material melts/solidifies. The main merits of usage the microcapsulated PCM slurries are as follows: (i) the phase change temperature range could be well fitted for purposes of a specific system by properly selecting the PCM, (ii) the slurry particle size can be very small, which results in smaller frictional pressure loss for the same mass flow rate and less risk of clogging the transportation pipes, (iii) the extremely sharp viscosity decreasing in the bimodal diameter distribution dispersions, which sufficiently increases the pumping efficiency of the concentrated slurries.
It was found that nonencapsulated PCMs are sticky and can glue together to form large lumps; clogging often occurs in a piping system, resulting in failure to circulate the slurry through the system, so PCM encapsulation are recently widely used. The small (units of microns or less) PCM capsules were found a very stable during pumping, repeated circulations through a slurry flow circuit and multiply thermal cycling across the PCM melting temperature, therefore, this type of slurry can be treated as a conventional single-phase working fluid.